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Mixing valve

A mixing valve is a throttling valve upstream of a desalter that shears wash water into fine droplets so they meet the salty water in the crude. Its pressure drop balances mixing against making an emulsion too tight to break.

Also written as: mix valve

Mixing valve explained

Typical published pressure drops are roughly 0.3 to 1.5 bar. Too little gives poor salt removal because wash water and brine never meet. Too much gives high water carry-over and oily effluent. The optimum changes with crude type and is found by stepwise adjustment against outlet salt and BS&W.

  • Crude oil desalters

    A crude oil desalter removes salt from crude by mixing in low-salinity wash water to dilute the brine, then removing the water in an electrostatic coalescer.…

  • Electrostatic coalescers

    An electrostatic coalescer is a pressure vessel that uses a high-voltage electric field to merge small water droplets in crude oil into drops large enough to…

  • Crude oil treatment

    A crude oil treatment system removes water, salt and sediment so that crude meets its sales or refinery specification. It works as a train: separation, heating,…

  • Troubleshooting and performance diagnostics

    Troubleshooting a desalter, dehydrator or hydrogen pressure swing adsorption (PSA) unit starts with the symptom, because each symptom has a short list of probable causes and…

  • Process and detailed design

    Process and detailed design turns a requirement, often no more than a datasheet, into a complete package that a fabricator can build and an operator can…

  • Design review for capacity enhancement

    A design review for capacity enhancement establishes what actually limits an existing crude oil treatment train or hydrogen purification unit, and which changes remove that limit…

Crude oil desalter
A crude oil desalter is an electrostatic coalescer operated with wash water mixed into the crude upstream. The water dilutes the salty droplets in the crude, and the coalescer then removes the water and most of the salt with it.
Wash water
Wash water is low-salinity water injected into crude oil upstream of a desalter to dilute the salty water droplets it carries. It is typically added at 3 to 10 per cent of the crude volume, then removed in the desalter.
Electrostatic coalescer
An electrostatic coalescer is a pressure vessel with energised electrode grids, in which a high-voltage field coalesces water droplets in crude oil so they settle. Used without wash water it is a dehydrator; with wash water it is a desalter.
Pounds per thousand barrels (PTB)
Pounds per thousand barrels (PTB) is the customary unit for the salt content of crude oil: pounds of salt, expressed as sodium chloride, in one thousand barrels of crude. One PTB is about 2.85 milligrams per litre.
Demulsifier
A demulsifier is a surface-active chemical injected into crude oil at parts-per-million levels to displace the natural films that stabilise water droplets. With the films weakened, droplets can merge and settle. It works together with heat, time and electrostatic fields.
Interface level
Interface level is the height of the boundary between settled water and the oil above it in a separator, dehydrator or desalter. Controlled by the water outlet valve, it must sit below the electrodes and above the water outlet.

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